@ben explained about the 'overheating' situation in the previous pages..
from my layman point of view, the console will still run hot.. i mean, there's a lot of graphic processing running inside of it..
whether it's overheating or not - i'm sure that there are already failsafe built inside to avoid overheating and should be safe to use.
however, if you play the game and suddenly the game pauses, with the warning "your PS4 is running too hot, please turn-off bla bla bla",
then that is surely a sign that your system is indeed overheating.
another thing to point - i also put it in the tv cabinet, where air is restricted.
hence, i need the help from my trusty table fan to 'stir things up' for the console to let it cool down better.
before this, i put the PS4 on top of the cabinet with a vertical stand, it can run slightly less noisy
and no issue with the overheating warning.
i'm thinking that a lower temperature will increase its life span. and i do not have a table fan, just air conditioning.